The present invention is directed to a transponder and wavelength division-multiplexing optical transmission equipment for optical communication. More specifically, it relates to a transponder and wavelength division-multiplexing optical transmission equipment for transmitting wavelength division-multiplexed digital signals.
The explosive growth of the Internet has required various modes of data communication services over communications networks. Communication vendors, for example, are required to transmit various types of signals with different signal transmission rates, transmission formats, and protocols over a single optical network. Signal transmission modes for digital signals that are transmitted over wavelength division-multiplexing networks include, for example, Synchronous Digital Hierarchy (SDH), Synchronous Optical NETwork (SONET), Fast Ethernet (100Base FX), Giga Ethernet (1000Base SX, LX), and Fibre Channel (1.06 GBd, 2.12 GBd). In particular, metropolitan optical networks require a service-providing function capable of frequently changing the signal transmission mode for each wavelength and a circuit-providing function capable of freely changing the connectivity between a sender and a receiver to suit the user's (client) requirements.
Both SDH and SONET, which have principally been used for the transmission of voice signals over conventional networks, have a fixed frame format and fixed data rate signal transmission methods that include communication control overheads. In these transmission modes, the monitoring of transmission quality or monitor of transmission quality, alarm monitoring, and path control are performed by means of transmission quality control bits that are assigned to the frame format. The transmission quality is controlled through the continuous processing of designated signal parts that occur in the digital signal strings. Further, optical networks based on either SDH or SONET have assured high transmission quality at the digital signal level through the execution of “3-regeneration repeat” processing at each node, which regenerates received digital signals in terms of three elements—strength, waveform, and timing—and the relay of the regenerated signals.
Transmission of the multiplexed optical signals in the SDH, SONET, or other digital signal modes over a common optical network requires a maximum transparency, which means transmitting received optical signals in their original form to various nodes and transponders to the maximum possible extent. A high degree of transparency in optical networks enhances their service providing function, reduces the cost of the networks and improves the versatility of the networks. To maintain an optical network's transparency, during the detection of the “0” and “1” signal levels of client digital signals and their conversion into optical signals at a specific wavelength on a wavelength division-multiplexing network, the signal regeneration processing regenerates a signal strength or re-shaping and waveforms using a 2R regeneration. Alternatively, the signal regeneration processing regenerates the digital signal's clock timing in addition to the signal strengths and waveforms using a 3R regeneration.
2R regeneration does not reproduce any degeneration of signal timing and easily satisfies the digital signal transparency requirement. On the other hand, it does not guarantee any regeneration timing. This regeneration can produce temporal fluctuations in the signals that are regenerated and relayed over the network, which results in a reduction in transmission quality. By contrast, 3R regeneration requires the setting of a specific oscillation frequency that is appropriate for the signal transmission mode being used on the phase-locked loop or phase lock loop (PLL) circuit that regenerates the clock timing for received signals. Conventionally, the setting of an oscillation frequency on the PLL is performed externally by predetermining the optical signal transmission rate over the line connecting the client system to the transponder. In optical transmission using a point-multipoint configuration in the asynchronous transfer mode (ATM), a burst (intermittent) transmission method can be employed. In this method, downward signals from a point to multi-points use a continuous pattern. For the upward transfers, signals are transmitted from multiple points only when they are needed. The receiver of the burst signals uses a technique that automatically extracts synchronized regeneration signals from the signals. In such a clock synchronization extraction technique, however, the clock rate is based on the signals received from an upstream source. The synchronization of the burst signals received from a downstream source involves a clock phase synchronization. The problem to be solved in the present invention involves the synchronization of signals that have different clock rates. The synchronization technique contained in the ATM based solely on the use of phases, does not offer any information for the solution of the problem.
In an optical network system in which a transponder is installed between a client equipment and a node so that the changing of signal transmission mode and changing the connectivity between a sender and a receiver can be flexibly performed. In addition, the type of client equipment connected to the transponder is frequently changed. In such a case, it is difficult and costly to externally set a frequency for the PLL circuit for 3R regeneration through human intervention.